Polymerization of thermosetting resins requires a supply of heat. This has heretofore been provided by conduction, for example on a heating plate, by radiation, for example in an infrared tunnel, or by convection, for example in a stream of hot air. These heating means may also be used in combination; for example a conventional process for polymerizing a piece of thermosetting resin is to place it in an oven, in which case the piece is heated both by conduction, radiation and by convection.